1. Field of the Invention
The present invention relates to an optical semiconductor device capable of achieving both high-speed operation and a high output.
2. Background Art
With the marked improvement in optical communication speed, uses of optical semiconductor devices with a need for high-speed operation have increased. Also, there is a demand for a direct modulation type of semiconductor laser which is a distributed feedback semiconductor laser usable even at a high temperature, directly modulated at a high speed, and designed to realize high-speed operation at a low cost. It is necessary that this direct-modulation-type semiconductor laser have a reduced element capacitance for high-speed operation and be capable of obtaining a sufficient optical output even at a high temperature. A structure using a p-type semiconductor layer and a high-resistance semiconductor layer as a buried layer has therefore been proposed (see, for example, Japanese Patent Laid-Open No. 2011-249767).
Reducing the loss of light and carriers in an active layer, i.e., a laser emission region, and a region around the active layer is effective in increasing the output. Also, increasing the relaxation vibration frequency is effective in achieving high-frequency operation. An element using a modulation-doped structure having barrier layers doped with a p-type dopant in a multiple quantum well structure has therefore been proposed (see, for example, Japanese Patent No. 4517653).